1. Field of the Invention
The present invention concerns a method for preparing a fortified metal filter in which the pore ratio is precisely controlled, and the filter so-produced.
2. Prior Art
There is presently known a filter material which is obtained by first compressing and, then, sintering a web-like system of numerous fine metal wires or by sintering a metal powder. In particular, the filter material obtained by compressing the web-like system of fine metal wires has been widely applied because of its large pore ratio and strength compared to the filter material obtained from a metal powder.
Irrespective of the widespread growth of the filter material, the web-like filter material can still be improved upon, if the following can be achieved:
(1) A high filter efficiency and sufficiently large pore ratio can be maintained by compression; PA0 (2) The fusion strength of the sintered part can be substantially maintained; PA0 (3) The ease with which the pore ratio of the product is controlled; PA0 (4) A simpler method for producing the product; PA0 (5) Control of free particles arising from repeated heating compression.
In general, the filter efficiency is inversely proportional to the pore ratio due to the production techniques utilized. A filter material which satisfies both of these requirements can not be produced by the conventional methods.
A filter often causes exfoliation in the sintered part reducing the pore ratio and filter efficiency. Since this is not desirable for most purposes, a high strength is required in the sintered part. However, unlike filters prepared from powdered metal, sufficient fusion between wires cannot be attained regardless of the orientation and compression of each wire in the filter of fine metal wires.
According to the conventional procedures, the pore ratio of a filter is controlled by the degree of compression. But the pore ratio is not sufficiently altered by the degree of compression employed. Consequently, the pore ratio of the commerical product thus far available is not controlled accurately. The control of the pore ratio is necessary for evaluating the efficiency of a comparatively accurate filtering device.
The conventional methods for producing the filters hereof employ repeated compression after annealing. This is the procedure that has been used to control the pore ratio and the filtering efficiency of the product. However, if compression and annealing are not performed simultaneously, as seen in this case, not only is the heat efficiency low, but the diffusion of metal in the fused part is prevented by inefficient heat transfer and repeated heating compression. This occasionally causes a region of heterogeneity arising from massive fusion parts in the system.